The broad objective of this research is to obtain a basic understanding of the mechanisms which govern the functional life-span of the corpus luteum during the menstrual cycle and during early pregnancy in primates. Functional luteolysis in primates cannot be accounted for by a decline in a circulating luteotropic hormone or by the production of a uterine luteolysin. However, relatively small increases in circulating estradiol, achieved experimentally during the luteal phase of the menstrual cycle, induced premature functional regression of the corpus luteum of the rhesus monkey. This observation has led to the hypothesis that estrogens secreted during the course of the luteal phase of the menstrual cycle, normally precipitate functional involution of the corpus luteum. Experiments are being conducted to test this hypothesis. The objective of initial studies was to determine the site (ovary, uterus or hypothalamo-hypophysial axis) whereby estrogen acts luteolytically. The results indicate that the luteolytic action of estradiol in the monkey can be accounted for by an action of the steroid directly within the ovary containing the corpus luteum. Future studies will examine the biochemical mechanisms and physiological importance of that phenomenon. BIBLIOGRAPHIC REFERENCES: Karsch, F.J. and G.P. Sutton, An intra-ovarian site for the luteolytic action of estrogen in the rhesus monkey, Endocrinology 98:553, 1976. Foster, D.L. and F.J. Karsh, Inhibition of tonic LH secretion by progesterone in immature female sheep, Endocrinology 99:1, 1976.